Essential role of aralar in the transduction of small Ca2+ signals to neuronal mitochondria

Aralar, the neuronal Ca2+-binding mitochondrial aspartate-glutamate carrier, has Ca2+ binding domains facing the extramitochondrial space and functions in the malate-aspartate NADH shuttle (MAS). Here we showed that MAS activity in brain mitochondria is stimulated by extramitochondrial Ca2+ with an S-0.5 of 324 nM. By employing primary neuronal cultures from control and aralar-deficient mice and NAD(P) H imaging with two-photon excitation microscopy, we showed that lactate utilization involves a substantial transfer of NAD(P) H to mitochondria in control but not aralar-deficient neurons, in agreement with the lack of MAS activity associated with aralar deficiency. The increase in mitochondrial NAD(P) H was greatly potentiated by large [Ca2+](i) signals both in control and aralar-deficient neurons, showing that these large signals activate the Ca2+ uniporter and mitochondrial dehydrogenases but not MAS activity. On the other hand, small [Ca2+](i) signals potentiate the increase in mitochondrial NAD(P) H only in control but not in aralar-deficient neurons. We concluded that neuronal MAS activity is selectively activated by small Ca2+ signals that fall below the activation range of the Ca2+ uniporter and plays an essential role in mitochondrial Ca2+ signaling.